The study used Aedes aegypti mosquitoes, the species that carries Dengue fever in Australia.

"It has been responsible for outbreaks of Dengue fever in Cairns and Townsville," said Dr Dave Walter, lecturer in Arthropods and Human Health at the University of Queensland.

The US researchers put mosquitoes into tubes treated with either diethyl-meta-toluamide (DEET) or catnip oil (nepetalactone), then counted the insects left in the treated ends of the tubes after 10 minutes.

Catnip oil - which is extracted from the catnip plant through distillation - was more effective. It takes one-tenth as much catnip oil to have the same repellancy as DEET, they found.

Most commercial insect repellents use DEET in a concentration of 5 per cent to 25 per cent.

The researchers are now seeking a patent for the use of catnip oil as an insect repellent, having discovered two years ago that it also repels cockroaches.

According to Dr Walter, Aedes aegypti is an "unusual mosquito", preferring to live inside houses and bite people. Most biting insects, including other mosquitoes, prefer other animals to humans.

Outbreaks of Dengue fever were a problem for Australian troops in East Timor in 1999.

"There were about 700 cases of illness, of which half were Dengue fever," said Dr Walter. Most of the other half were malaria.

Dengue fever is a viral disease that brings on symptoms of headache and fever. There are several strains of the virus, of which Australia host to only one.

"The worst things happen if you get infected with two different strains of the virus," Dr Walter explained. A person who has been infected with Dengue fever in Australia who goes overseas and is bitten elsewhere can die from haemorrhagic fever.

"The mortality rate is very high under those circumstances."

Aedes aegypti also carries yellow fever, which does not exist in Australia.

Many substances are known to repel insects. Some work by masking the natural odour of the host, so the insect doesn't recognise them as a food source. Others are distasteful to the insect.

"Often repellents are quite specific to the insects," said Dr Walter - so this result does not necessarily mean that catnip oil will be a universal alternative to DDET.

"A lot of people won't use DEET compounds, but it is the best way to save your life if you are in an area with a nasty arthropod disease," he cautioned.

In addition, catnip oil is yet to be tested to ensure it is safe for human use.

"Remember that nature produces a lot of things that sound good but are dangerous at high doses," warned Dr Walter.

Catnip is a perennial herb belonging to the mint family, and grows well in Australia.